dashboard: add gobuilder -fail mode

dashboard: add gobuilder -fail mode

This is for filling a column with "fail", like I just did for
Windows, when the builder would get stuck running that
build.  (We have safeguards against the tests getting stuck
but this was the bootstrap build getting stuck.)

I usually use -cmd=/bin/false, but this avoids the Mercurial
checkouts, which means it runs instantly instead of requiring
~1 minute per "fail".

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5649049